Anisotropic rare earth-based permanent magnet material

Details Anisotropic rare earth-based permanent magnet material Anisotropic rare earth-based permanent magnet material

2001-12-18

2003-07-08

Sheehan, John (Department: 1742)

Metal treatment

Stock

Magnetic

C420S083000, C420S121000

Reexamination Certificate

active

06589367

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a magnetically anisotropic rare earth-based permanent magnet material prepared by a unique method.
Rare earth-based permanent magnets currently under large-scale industrial mass production are classified into two types including Sm/Co-based magnets and Nd/Fe/B-based magnets, of which the demand for the magnets of the latter type is rapidly growing year by year in respects of their outstandingly high magnetic properties and relatively low material costs thereof as compared with the magnets of the former type.
Among several manufacturing methods heretofore developed for the preparation of the Nd/Fe/B-based permanent magnets, the most widely industrialized method is the so-called sintering method. The Nd/Fe/B-based permanent magnets prepared by this method have a metallographic structure of which the principal phase is the magnetically hard Nd
2
Fe
14
B phase accompanied by a phase having a higher neodymium content than that, referred to as the Nd-rich phase hereinafter, and the Nd
11
Fe
4
B
4
phase, referred to as the B-rich phase hereinafter.
In the sintering method for the preparation of the above described Nd/Fe/B-based magnets, an alloy ingot having a composition of which the contents of neodymium and boron each in some excess over the stoichiometric proportion of the respective elements corresponding to the composition of the formula Nd
2
Fe
14
B is a finely pulverized into a fine powder having a particle diameter of a few micrometers and the powder is compression-molded in a magnetic field into a powder compact consisting of the alloy particles having their easy magnetization axis aligned in the direction of the magnetic field followed by a heat treatment of the powder compact for sintering at about 1100° C. and aging of the sintered body at a lower temperature (see, for example, M. Sagawa, et al., Japanese Journal of Applied Physics, volume 26, 1987, page 785). The magnet thus obtained is a magnetically anisotropic permanent magnet having a high coercive force exhibited by virtue of the interface cleaning effect of the Nd-rich phase surrounding the grains of the principal phase of Nd
2
Fe
14
B.
Alternatively, the alloy powder can be obtained by pulverizing a quenched thin magnet alloy ribbon prepared by the so-called melt-span method in which a melt of the magnet alloy is ejected at the surface of a rotating quenching roller to be rapidly solidified thereon into the form of a thin ribbon (see, for example, R. W. Lee, Physics Letter, volume 46, 1985, page 790). The quenched thin ribbon of the magnet alloy prepared by the melt-spin method has a structure, like the alloy ingot prepared by casting of a melt, consisting of the Nd
2
Fe
14
B phase as the magnetically hard principal phase but the grain diameter of this principal phase is much smaller than in the sintered magnets to be about the same order as the single magnetic domains in the range from 20 to 100 nm.
The quenched thin magnet alloy ribbons can be processed into permanent magnet blocks in three different ways. In the first method, a fine powder of the quenched thin ribbons is blended with a resin as a binder and the blend is shaped into the form of a magnet block of so-called resin-bond magnets. Although the process for the preparation of resin-bond magnets is simple and inexpensive, the resin-bond magnet is necessarily magnetically isotropic and the impregnation density of the magnetic particles is relatively low so that resin-bond magnets cannot be very excellent in the magnetic properties. In the second method, a powder of the quenched thin ribbons is shaped in a hot press to give a magnetically isotropic permanent magnet. In the third method, the isotropic bulk magnet obtained by the second method is further subjected to a hot-working treatment under a compressive force to have the magnetic particles with the easy magnetization axis aligned in the direction of compression (see, for example, Japanese Patent Kokai 60-100402).
On the other hand, extensive development works are now under way in order to accomplish further upgrading of the magnetic properties of rare earth-based permanent magnets of the next generation including so-called nanocomposite magnets highlighted in recent years (see, for example, E. F. Kneller, et al., IEEE Transaction Magnetics, volume 27, 1991, page 3588).
The metallographic structure of the rare earth-based nanocomposite permanent magnets is quite different from that of the conventional sintered magnets. Namely, the conventional magnets have the magnetically hard principal phase of Nd
2
Fe
14
B but are free from magnetically soft phases of, for example, bcc-Fe and Fe-rich phases such as Fe
3
B, Fe
2
B and the like. In contrast thereto, the nanocomposite magnets have a structure consisting of magnetically hard and magnetically soft phases finely dispersed each in the other in fineness of an order of several tens nanometers, in which an exchange coupling of magnetization is exhibited between the magnetically hard and soft phases inhibiting reversal of magnetization of the magnetically soft phase leading to a behavior of the whole magnet body as if it consists of a single magnetically hard phase. This principle of nanocomposite magnets provides a possibility of obtaining a greatly increased saturation magnetization without adversely affecting the coercive force even as a combination with other known materials. According to the result of theoretical calculation reported by R. Skomski, et al. in Physical Review B, volume 48, 1993, page 15812, the possible largest value of the maximum energy product (BH)
max
is 137 MGOe for the system of Sm
2
Fe
17
N
3
/(Fe,Co).
Several reports are available on the actual preparation of rare earth-based nanocomposite permanent magnets including R. Coehoorn, et al., Journal de Physique, volume 49, 1988, page C8-669, for the Nd
2
Fe
14
B/Fe
3
B magnets, Japanese Patent Kokai 7-173501 and 7-176417 and L. Withanawasam, et al., Journal of Applied Physics, volume 76, 1994, page 7065, for Nd
2
Fe
14
B/Fe magnets and J. D. Ding, et al., Journal of Magnetism and Magnetic Materials, volume 124, 1993, page L1, for Sm
2
Fe
17
N
3
/Fe magnets.
Each of the methods disclosed in these reports and patent documents for the preparation of the nanocomposite magnets utilizes microcrystallization by the heat treatment of a powder of a quenched thin alloy ribbon prepared by the melt-spin method or an amorphous alloy prepared by the mechanical alloying method so that alignment of the magnetic grains cannot be accomplished likewise in the above described resin-bond magnets not to give a magnetically anisotropic high-performance permanent magnet.
While attempts were made for the preparation of a bulk magnet of nanocomposite structure by utilizing a hot press as reported in J. Wecker, et al., Journal of Applied Physics Letter, volume 67, 1995, page 563, M. Jurczyk, et al., Journal of Alloys and Compounds, volume 230, 1995, page L1, Kojima, et al., Synopsis of 21st Scientific Lectures of Japan Applied Magnetics Society, 1997, page 384, and elsewhere, the magnets obtained by this method are close to a magnetically isotropic magnet as being little imparted with magnetic anisotropy.
Thus, it is the present status of the magnet technology that no industrially applicable method is reported for the preparation of a magnetically anisotropic nanocomposite permanent magnet notwithstanding the eager desire to obtain rare earth-based high-performance permanent magnets which can be expected according to the results of the theoretical calculation.
SUMMARY OF THE INVENTION
The present invention accordingly has an object to provide a method for the preparation of a high-performance rare earth-based magnetically anisotropic permanent magnet having a nanocomposite metallographic structure consisting of a magnetically hard phase and a magnetically soft phase finely dispersed each in the other, of which the grains of the magnetically hard phase are aligned relative to the easy magnetization axis.
Thus, the present invention provides a method

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